β-Lapachone (β-lap) is a potent cytotoxic agent that demonstrates antitumor activity against a variety of human cancer cells at concentrations typically in the range of 1-10 μM (IC50). The drug was first isolated from the bark of the Lapacho tree (genus Tabebuia) in the rainforests of South America, which has a long history as an herbal medicine. Cytotoxicity has been demonstrated in transformed cell lines derived from patients with promyelocytic leukemia, prostate, malignant glioma, hepatoma, colon, breast, ovarian, pancreatic, and multiple myeloma cell lines including drug-resistant lines (Planchon et al., Cancer Res., 55 (1996) 3706; Li, C. J., et al., Cancer Res., 55 (1995) 3712; Weller, M. et al., Int. J Cancer, 73 (1997) 707; Lai, C. C., et al. Histol Histopathol, 13 (I 998) 8; Huang, L., et al., Mol Med, 5, (1999) 711; Wuertzberger, S. M., et al., Cancer Res., 58 (1998) 1876; Li, C. J. et al., Proc. Natl. Acad Sci. USA; 96(23) (1999) 13369-74; Li, Y., et al., Mol Med, 6 (2000) 1008; and Li, Y. Z., Mol Med, 5 (1999) 232). No cytotoxic effects were observed on normal fresh or proliferating human PBMC (Li, Y., et al., Mol Med, 6 (2000) 1008).
β-Lapachone and its derivatives have also been synthesized and tested as anti-viral and anti-parasitic agents (Goncalves, A. M., et al. . Mol. Biochem. Parasitology, I (1980) 167-176; Schaffner-Sabba, K., et al., J Med, Chem., 27 (1984) 990-994).
β-Lapachone has been shown to be a DNA repair inhibitor that sensitizes cells to DNA-damaging agents including radiation (Boothman, D. A. et al., Cancer Res, 47 (1987) 5361; Boorstein, R. J., et al., Biochem. Biophys. Commun., 117 (1983) 30). β-Lapachone has been asserted to have potent in vitro inhibition of human DNA Topoisomerases I (Li, C. J., et al., J Biol. Chem., 268 (1993) 22463) and II (Frydman, B. et al., Cancer Res. 57 (1997) 620) with novel mechanisms of action. Topoisomerase I is an enzyme that unwinds the DNA that makes up the chromosomes. The chromosomes must be unwound in order for the cell to use the genetic information to synthesize proteins; β-lapachone may keep the chromosomes wound tight, so that the cell cannot make proteins. As a result, the cell stops growing. Because cancer cells are constantly replicating and circumvent many mechanisms that restrict replication in normal cells, they are more vulnerable to topoisomerase inhibition than are normal cells.
Another possible intracellular target for β-lapachone in tumor cells is the enzyme NAD(P)H:quinone oxidoreductase (NQO1, E.C. 1.6.99.2). β-lapachone is bioactivated by the NQO1 enzyme, which is a ubiquitous flavoprotein found in most eukaryotic cells. This enzyme catalyzes a two-electron reduction of various quinones, utilizing either NADH or NADPH as electron donors. Biochemical studies suggest that reduction of β-lapachone by NQO1 leads to a “futile cycling” between the quinone and hydroquinone forms with a concomitant loss of reduced NADH or NAD(P)H (Pink, J. J. et al, J Biol Chem., 275 (2000) 5416). The exhaustion of these reduced enzyme cofactors may be a critical factor for the activation of the apoptotic pathway after β-lapachone treatment. The human NQO1 gene encodes a 30 kDa protein that is expressed in a tissue-dependent manner. More importantly, NQO1 is over-expressed (up to 20-fold) in a number of tumors, including breast, colon and lung cancers, compared with adjacent normal tissue (1-4). Over-expression of NQO1 in cancerous cells makes it an ideal target for tumor-selective drug therapies with minimal toxicities to healthy cells.
Despite the potency and selectivity of β-lap in killing cancer cells in vitro, the low water solubility of β-lapachone (0.04 mg/ml or 0.16 mM) limits its potential for systemic administration and clinical applications in vivo. β-lapachone is highly insoluble in water and has only limited solubility in common solvent systems used for topical and parenteral administration. As a result, there is a need for improved formulations of β-lapachone for therapeutic purposes that are both safe and readily bioavailable to the subject to which the formulation is administered.